Whisk the sugar and the cornstarch in a microwaveable glass mixing bowl or a 2 or 4 cup glass measuring cup. Whisk in the milk until smooth and microwave on high for 3 1/2 minutes. Whisk again. Microwave an additional 2 or 3 times for 30-45 second intervals, whisking after each interval, until the mixture thickens to the consistency of a thin gravy or sauce. Whisk in the cold butter until completely dissolved and smooth. Fold in the desired amount of lemon zest, cover with plastic wrap pressed directly on the surface, cool and refrigerate. Will keep for at least 5 days. If some of the liquid separates, simply whisk until smooth. Serve cold or at room temperature.